Communication apparatus



April 1951 R. H. CAMPBELL 2,547,855

COMMUNICATION APPARATUS Filed May 7, 1948 I I a l lrOl INVENTOR. RichardH Campbell B Y M ru/mwwzmwa/ w .through the central station under thePatented Apr. 3, 1951 COMMUNICATION APPARATUS Richard R. Campbell,Racine, Wis., assignor to Webster Electric Company, Racine, Wis., acorporation of Delaware Application May '7, 1948, Serial No. 25,752

21 Claims.

The present invention relates to communication apparatus andparticularly to intercommunicating apparatus of the loud-speaking type.

One of the primary objects of the present invention is to provide a newand improved communication apparatus comprising a central station and aplurality of remote or substations wherein communication can be carriedon between the remote stations through means including amplifying meanslocated at the central station.

A further object of the present invention is to provide a new andimproved communication system comprising a central station and aplurality of remote stations wherein communication can be carried onbetween the central station and selected remote stations through aselected one of a plurality of amplifying means at the central station.

Another object of the present invention is to provide a new and improvedcommunication. system comprising a central station and a plurality ofremote stations wherein communication can be carried on between remotestations control of one of the remote stations.

A further object of the present invention is to provide new and improvedcommunication apparatus and signaling arrangements for systems of thecharacter set forth above.

A still further object of the present invention is to provide new andimproved communication system including central and remote stationswherein signaling circuits and means are utilized for other purposes,such as controlling communication, thereby to reduce the number ofcomponent parts and expense.

Other objects and advantages of the present invention will becomeapparent from the ensuing description of an embodiment thereof, in thecourse of which reference is had to the accompanying drawings, thesingle figure of which is a diagrammatic representation of communicationapparatus embodying the invention.

The invention is applicable to systems comprising a number of stations,although but four have been illustrated in the figure. The illustratedstations include three remote stations I, 2 and 3 and a central station4, at which most of the equipment is located. It should be understood,however, that the number of stations may be changed as desired.

The remote stations include translating devices ll, l2 and i3 havingtheir voice coils connected to associated interstation communicationcircuits 2|, 22 and 23, respectively. The remote stations are alsoconnected to the central station by the signaling circuits 3!, 32 and33, each of which includes a switch 34 utilized both for signaling andcommunication direction control, as will be described in greater detailhereinafter. The switch is normally open, to which position it may bebiased by a, spring, not shown, and one terminal is grounded, as readilyapparent from the figure.

The central station includes control switches and circuits enabling (1)the remote stations to signal the central station, (2) the centralstation to be placed in two-way communication with selected remotestations, the direction of communication being under the control of thecentral station, (3) the central station to call any desired remotestation and have two-way communication with it, and (4) the centralstation to establish communication circuits between selected remotestations and preferably so that the direction of communication is underthe control of one of the remote stations between which communication isestablished. The central station includes a plurality of amplifyingmeans and controls enabling the interchange of functions performed bythe amplifying means so that should it be desirable, for instance, asthe result of failure of one amplifier, to substitute another, this canbe done.

Communication may be established between the central station and theremote stations through station selector switches i! 42 and 43individual, respectively, to stations I, 2 and 3. When any of theseswitches is closed, the corresponding interstation communication circuit2!, 22 or 23 is connected to a central station two-way communicationcircuit 44 by two lowermost switch blades of the respective stationselector switches, which are indicated by the suffixes l and 2 after thestation selector switch reference character.

Two-way communication between the central station and selected remotestation or stations is carried on through a transmission amplifier 46and a reception amplifier 48, under the control of the operator at thecentral station. The communication circuit is is normally connected tothe input of the amplifier 48 through a switch 59 operable into a secondposition to connect the circuit 54 to the output of the transmissionamplifier 46. The switch 56 is operable by a relay 52 adapted to beconnected to a suitable source of power, not shown, by the closure of aswitch 54. When thus connected to the source of power, the switch 56 isoperated from its indicated or reception position to a second ortransmission position.

In the indicated reception position of the switch, the communicationcircuit 44 (and the selected interstation circuit) is connected to theinput of amplifier 48 through a reception circuit 56, the two switchblades 58 of a selectively operable change-over switch, indicated in itsentirety by reference character 60, and the input connections 62. Theoutput of the amplifier is connected to a translating device, which maybe a loudspeaker 64 through the output connections 66, switch blades 68of switch 60, circuit I a selector switch I2, the purpose of which willbe described further hereinafter, and a volume controlling potentiometer14 adjustably connected to the voice coil of loud-speaker 64.

In the transmission position of switch 50, the circuit 44 is connectedto the output connections 16 of amplifier 46, to the input of which isconnected, preferably permanently, a transmitting means which may be amicrophone I8.

Reviewing the mode of communication between the central and remotestations, it may be noted that this is done under the control of theswitch 54 and subsequent to establishment of a circuit to the selectedremote station or stations through the closure of switches 4 I, 42 or43.

In the event amplifier 48 should fail or should it be desired not to useit for any reason whatever, another amplifier 80 can be substituted forit in the reception circuit. Before describing how this is done, it maybe well to note that the amplifier 80 is normally used for communicationbetween selected remote stations in a manner to be described in detailhereinafter. When, however, amplifier 80 is used for reception in thesame manner as amplifier 48, its input connections 82 are connected tothe reception circuit 56 through switch blades 84 of switch 60 and itsoutput connections 86 are connected to the loud-speaker 64 throughswitch blades 88 of switch 60, circuit 90, switch I2 in the second ofits two positions, and potentiometer I4.

The above-described mode of communication between the central and remotestations obtains whenever the station selector switches for the remotestations are operated. These switches are operated by the operator atthe central station either when he desires to call a remote thearrangement is such that the remote station can signal the centralstation of its desire to communicate with the central station. Thesignal is given at the central station upon closure of switch 34, whichclosure may be momentary in nature. Once the switch is closed, a relayIOI, I02, or I 03, corresponding to stations I, 2, or 3, is operated andlocked in operated position through a locking circuit to be describedshortly. When the relay operates, a pilot lamp I04 individual to thestation is lighted and a buzzer I06, common to all the stations, sounds.The light I04 is connected across a suitable source of power which maybe the secondary winding I06 of a transformer I08, the primary windingIIO of which may be connected to a suitable source of power, not shown.The connection of the pilot light to the secondary winding is effectedby a relay operated switch H2 and a circuit including conductors H4 andH8. The buzzer I06 is in series with conductor II6 so that its c r u tis also closed or completed when the switch H2 is closed.

The signaling arrangement is such that even momentary closure of aswitch 34 locks the relay corresponding to the particular station orstations at which the signaling switch is closed. When a switch 34 isclosed, the relay winding I I8 is connected across a suitable source ofpower which may be the battery I 20 at the central station through aground connection, the interstation signal circuit 3|, 32, or 33, andconductor I22, which is connected to and common to all the relaywindings and leads to the battery. The relay is locked in operatedposition through a locking circuit including conductor I24 (here, again,like reference characters are applied to like parts associated with therelays and their respective locking circuits), switch blade I26 operableby the relay to complete the locking circuit, conductor I 28. switchblade I30 operable simultaneously with the selector switch andconductors I3I, I32 and I33, individual to stations I, 2 and 3,respectively, and interrupted circular contact I34 of a locking circuitcontrol switch I36, the purpose of which will be described more fullyhereinafter, and ground conductor I38. The ground conductor and thus thelocking circuit is normally connected to the contact I34 common to thevarious locking circuits so that the relay locking circuits arecompleted whenever the relays are operated. However, the contact I34 isinterrupted so that when the switch I36 is operated, as it is uponestablishment of communication between selected remote stations, therelay IOI, I02, or I03 at one of the selected stations and the switch 34at that station may be used for controlling communication between theselected remote stations.

When the call from a remote station is noted, the selector switchcorresponding to that station is closed, whereupon the switch I30 opensthe locking circuit and the corresponding relay IOI, I02, or I03 isreturned to its normal position. When the communication circuit has beencompleted, communication can be carried on between the central stationand the selected re mote station in the manner described above inconnection with the initiation of a call at the central station. Certainfeatures of the above described portion of the system are disclosed andclaimed in my co-pendin application Serial No. 6,608, filed February 6,1948.

One of the important features of the present invention resides in theprovisions made for placing two of the remote stations in communicationwith each other and preferably with the direction of communicationcontrolled by the operator at one station, preferably the callingstation. A further feature resides in the use of the signaling switch 34for the purpose of controlling the direction of communication.

When one of the remote stations desires to communicate with another, thestation calls the central station in the manner described above. Thecentral station answers the call and when informed of the desire of thecalling remote station to call a called remote station, the centralstation operator establishes the necessary circuits. In describing thesecircuits, it is assumed that station 3 desires to communicate withstation I. The operator at station 3 calls the central station asalready described. The central station operator then calls station I ina manner also already described. When the arranse s ve been mace withoperators at stations I and 3.. th selector switches 41 and 43corresponding to the two stati ns which are closed for communicationbetween the remote stations and the centr l station. are op n n otherselectively operable switches; at the central station are operated toestablish the necessary communication and control circuits. Theseselectively operable switches. at the central station include amultipole switch, indicated as a whole by reference character Isll, andanother mul ipole switch, indicated as a whole by reference characterI42. The switch I40 is operated to a position corresponding to thecalling, s ation. in this case station 3, and. the switch I42 is;operated to a position corresponding to the called station, station I.When these switches are operated as indicated, the loud-speakers II andI3 at stations I and 3 are connected hrough communication circuits to bedescribed, the switches I40 and H82. and selectively by a, om-.munication direction control switch I44 to the input or output ofamplifier 80. The switch I44 normally connects the calling station tothe output and. the called station to the input of the amplifier.direction of communication, the callin station operates switch 34,thereby to energize a relay winding I46 operating switch I44 to move thelatter to its other position and thereby reverse the connections o theamplifier.

The communication between selected remote stations is carried on throughtheir respective interstation communication circuits 2|, 22, 23, whichare connected at the central station by circuits I5I, I52 and I53,respectively, to con tacts individual to the remote stations and en,gageable by the movable switch blades I54 and I56 of switch I40 andswitch blades I58 and I 60 of switch I42. As illustrated, there arethree When it is desired to reverse the o contacts associated with eachswitch blade and these correspond to stations I, 2 and 3, respectively,starting from the lowermost contact.

Switch I40 is, as already indicated, operated to a positioncorresponding to the calling station, 1. e., station 3. It connectsstation 3 to the output of amplifier 00 through the circuit I62, switchblades I64 of switch I44, circuit 90. switch blades 88, and conductor66. The input of the amplifier 80 is connected to station I, the calledstation, through conductor 82, switch blades 04 of switch 60, circuitI66, switch blades I68 of switch I44, and circuit I10, the twoconductors of which are connected to switch blades I58 and I60. Thecommunication circuit is thus conditioned for communication from station3 to station I.

To reverse the direction of communication, the operator at station 3closes switch 34. This results in the connection of windin I46 to thebattery and in the operation of switch I44 to reverse the connections ofcircuits I62 and I10 to the amplifier so that station 3 is connected tothe input and station I to the output. The closure of switch 34 effectsenergization of a relay winding I12 by connection thereof across thebattery. One end ofthe Winding is connected directly to the batterythrough conductor I14. The other end is connected to the battery throughthe ground circuit and the circuit including conductor I16, a switchblade I18 movable simultaneously with switch blades I56 and I54 which,when station 3 is the calling sta tion, is in engagement with itsuppermost 31550-9 ciated ontact, conductor IBI, conductor 33, an switch34. It will thus be noted that windin I1? is, in efiect. connectedparallel with wind.- ins, N8 f relay- 03. corresponding. to stat on. 3.Th r ult w uld be h -l ckin of rel y I03 in operated position, but thisis prevented by the switch I36 which. breaks the locking circuit for therelay when the switch I40, of which switch I36 also forms a part, ismoved into the proper position for station 3. In other positions ofswitch I40, circuits are comple'tedthrough conductors I82 and I83 to theinterstation circuits 3| and 32 of stations I and 2.

When the winding I12 is energized by closur of switch 34 at station 3-,an energizing circuit is completed for winding I44. This, energizingcircuit extends from the battery through conductor I14, conductor I84,switch I86 operated by the winding I12, conductor I88, and the groundreturn conductor I90. 7 It should perhaps be noted that while it ispreferred that switch I40 be operated to a position corr spond n o the aling station, it cou d be operated to a position correspondin to hecalled station. In this event, the called station would, however, haveto control the direction of communication because the locking circuitfor its associated relay is broken y e switch I36.

The operator at he cen al s ation is iv n an ind cation tha the r m tetation are. c mmun eating with each oth r by the in tt flashing of thelight I04 corresponding to the callin station and by operation of thebuzzer I06. H w-v ever, the latter can be short circuited by the switchblade I92 of a switch I94 adapted to short circuit the buzzer when theswitch is closed. To indicate the-short circuiting of the buzzer, thereis provided a light I96 which is energized through a circuit includingthe switch blade I08 whenever the switch I94 is closed.

The operation of the system as a whole will now be reviewed. It will beassumed first that the central station desires to communicate with oneof the remote stations, says, station I. Station I is selected byclosure of station selector switch 4|. The switch 54 is closed tooperate switch 50 to its transmission position. The output of thetransmission amplifier 46 is thus connected to loud-speaker II atstation I. Consequently, the central station can call the operator atstation I. To receive from stationv I, the switch 54 is opened andswitch 50 returns to its indicated position wherein loud-speaker II isconnected to the in.- put of amplifier'il8 and the outputof the latteris connected to loud-speaker 64, as previously described.

When the central station and remote station I are communicating, otherstations can call the central station. For example, station 3 can do soby closure of switch 34 thereat. The result is the operation of relayI03, which effects lighting of lamp I04, the operation of buzzer I06,and which also looks itself in through the locking circuit includingswitch I30 of the selector switch 43 and the switch I36. As soon as thecentral station is through talking with station I, the selector switchII is opened and the selector switch 43 is closed. The central stationcan then talk to the remote stationn it is d ve e that station 3 desiresto communicate with staion I, hen. he c ntral station call st t a bfore. either befor or a er o enin th selector switch 43 ior station 3.

The cent al station can es ablish communicat tion circuit between thecall ng stat on and t e called station This s clone by operati g switchI40 m ses? its uppermost contac and opera ating switch I42 to engage itslowermost effective contact. This connects station 3 to the output ofamplifier 80 and station I to the input. To reverse the connections ofthe stations to establish communication in the opposite direction,switch 34 at station 3 is closed. This efiects energizing of Winding I46and operation of the switch I44 to its other position. The operation ofswitch 34 does not effect the locking in of relay I 03 because thelocking circuit for the latter is broken by switch I36.

The flashing of light I04 responsive to operation of switch 34 indicatesthat stations I and 3 are in communication with each other. This is alsoindicated by the buzzer I06 which, however, can be shorted by switchI94. When the buzzer is shorted, light I96 is also energized thereby toprovide an additional indication that remote stations are or have beenin communication with each other and that the buzzer has been shorted.

While stations I and 3 are in communication with each other, otherremote stations, such as station 2, can call the central station. Thecall will be indicated by the lighting of pilot lamp I04- and thelocking in of relay I02 just as when station 3 called station I. Therelay will be locked in because the locking circuit for relay I02, whichincludes conductor I32, is completed through switch I36. The centralstation can communicate with station 2 while stations I and 3 arecommunicating with each ther. The communication is carried on throughthe translation amplifier 46 and the reception amplifier 48 in a manneralready described.

The central station can also monitor the communication between stationsI and 3. To do this the switch I2 is moved to its other position whereinit is connected to conductor 90 which is connected to the output ofamplifier 80.

After the completion of a call between remote stations, the switches I40and I42 are returned to their indicated normal positions.

Should amplifier 48 fail or should it not be desired to use it for anyreason, the amplifier 80 can be substituted for it by operation ofswitch 60 to its second position, which is one position below itsindicated position. This also effects the change-over of amplifier 48for use in communication between remote stations. If amplifier 80 isused as a reception amplifier, then switch I2 is moved to its otherposition to connect it to the output of the amplifier through circuit90.

While the present invention has been described in connection with thedetails of a single embodiment thereof, it should be understood thatthese details are not intended to be limitative of the invention exceptin so far as set forth in the accompanying claims. The illustratedsystem may include more stations than indicated, and, if desired, thecentral station may be provided with a number of amplifierscorresponding to amplifier 80 so that communication can be carried onsimultaneously between more than two sets of stations.

Having thus described my invention, what I claim as new and desire tosecure by Letters Patent of the United States is:

1. Communication apparatus including, a first station having atranslating device utilizable both as a receiver and a transmitter, asecond station including transmission amplifying means, transmittingmeans connected to said transmission amplifying means, a pair ofreception amplifying means, reception means, selectively operable meansfor connecting said reception means to the output of one of saidreception amplifying means, an interstation circuit interconnecting saidstations, and selectively operable switch means connected to saidcircuit operable selectively to connect said circuit to saidtransmission amplifying means or to the input of the receptionamplifying means to the output of which said reception means isconnected.

2. Communication apparatus including a communication circuit, atransmission circuit, amplifying means having its output connected tosaid transmission circuit, transmitting means connected to the input ofthe amplifying means, a reception circuit, a pair of amplifying means,reception means, means including first selectively operable meansassociated with said communication, reception and transmission circuitsfor selectively connecting said communication circuit to said receptionor transmission circuits, and means including second selectivelyoperable means associated with said reception circuit, amplifying meansand reception means for connecting said reception circuit and receptionmeans to the input and output, respectively, of one or the other of saidpair of amplifying means.

3. A communication system including a plurality of remote stations, acentral station including a plurality of amplifying means, a firstcommunication circuit between each of said remote stations and thecentral station, means including station selector switches at thecentral station for selectively connecting said communication circuitsto one of said amplifying means for effecting communication between saidcentral and remote stations, a second communication circuit between eachof said remote stations and said central station, and means includingselectively operable switches at the central station for connecting saidsecond communication circuits to another of said amplifying means foreffecting communication between said remote stations.

4. A communication system including a plurality of remote stations, acentral station in cluding a plurality of amplifying means, a firstcommunication circuit between each of said remote stations and thecentral station, means including station selector switches at thecentral station for selectively connecting said communication circuitsto one of said amplifying means for effecting communication between saidcentral and remote stations, a second communication circuit between eachof said remote stations and. said central station, means includingselectively operable switches at the central station for connecting saidsecond communication circuits to another of said amplifying means foreffecting communication between said remote stations, and meansincluding selectively operable switches associated with the amplifyingmeans for reversing the positions of said amplifying means in said firstand second communication circuits.

5. A communication system including a plurality of remote stations, acentral station including a plurality of amplifying means, a firstcommunication circuit between each of said remote stations and thecentral station, means including station selector switches at thecentral station for selectively connecting said communication circuitsto one of said amplifying means for effecting communication between saidcentral and remote stations, a second communication circuit between eachof said remote stations and said central station, means includingselectively Operable switches at the central station for connecting saidsecond communication circuits to the other of said amplify-ing means,other selectively operable switches at said central station forreversing the connections ofsaid second communication circuits to saidother amplifying means, and control meansat one of said remote stationsfor controlling said other selectively operable switches for effectingcommunication between said remote stations.

"6. Communication apparatus includingya first station, a second station,a signaling circuit between said stations, a communication circuit be'-tween said stations, signaling switch means at the first stationoperable to a predetermined-position to changethe condition of saidsignaling circuit, relay means at the second stationconnected to saidsignaling circuit and operable in response tosaid change in condition ofsaid circuit, means controlled by said relay means and operable therebyfor maintaining said relay means in operatedcondition subsequent tooperation of said switch means at said first station from its saidpredetermined position, station selector switch means at said secondstation associated withsaid communication circuit including switch meanscontrolling said relay means operable from a (first to a second positionfor returning said relay means to a nonoperate'd conditiornand aselectively operable switch at said second station in circuit with saidstationselector switch means in its first position.

7. Communication apparatus including a first station, a second station,a normally open signaling circuit between said stations, a normally opencommunication circuit between said stations, a switch at the firststation closable to complete said signaling circuit, a relay atthesecond station in said signaling circuit energizable upon completion ofsaid circuit, arelay locking. circuit and a switch therein operable bysaid relay for maintaining said relay operated subsequent to opening ofsaid switch at said first station, a station selector switch at saidsecond station oper able to complete said communication circuitandincluding a normally closed switch in circuit with said relay operatedlocking circuit switch operable with said station selector switch toopen the relay circuit and deenergi'ze the relay, and a selectivelyoperable switch at said second station in said locking circuit adaptedto complete said ci'rcult in one position.

8. Communication apparatus including, a signaling circuit, acommunication circuit, relay means connected to saidsignaling circuitand operable in response to a momentary change in condition of saidcircuit, means controlled by said relay means and operable thereby forlocking said 1 relay means in operated condition, station selectorswitch means in said communication, circuit controlling said relay andoperable to a predetermined position for returning said relay means: toa nonoperated condition, and other selectively operable switch meansoperatively connected with said station selector switch, means andlocking means for rendering said locking means ineffective.

9. Communication apparatus including ,a signaling circuit, acommunication circuit,v a relay in said signaling circuit energizableuponmomentary completion of said circuit, a signal device, an energizingcircuit and a switch in said circuit for energizing said device uponsaid operation of said relay, a locking circuit .and a switch thereinoperable by. said: relay for maintainingsaid relay operated, "a stationselector switch operable to close said communication circuit, a normallyclosed switch in circuit with said locking circuit switch controllingsaid relay and thereby said signal device and operable With-saidselector switch for opening the relay circuit and deenergizing therelay, and a selectively operable switch in said locking circuitoperable to open said locking circuit.

:10. Communication apparatus including a plurality of first stations, asecond station, normally open signaling and communication circuitsbetween said second station and each of said first stations, a switch ateach of said first stations closable to complete its associatedsignaling circuit, a relay at the second station in the signalingcircuit for each of said first stations and energizable upon completionof its associated signaling circuit, a relay locking circuitfor each ofsaid re- :laysincluding a switch in said circuit operable by the relayfor maintaining said relay operated subsequent to opening-cf the switchat said first stations, station selector switches at said second stationfor each of said first stations operable to complete associatedcommunication circuits and each including a normally closed switch inthe locking circuit for the relay associated with the stationcorresponding to the selector switch, and manually operable switch meansnormally closing the locking circuits for all said relays and operableto open the locking circuit of a selected relay.

11. Communication apparatus including a plurality of signaling circuits,a plurality of communication circuits, relay means in each of saidsignaling circuits operable upon momentary completion of itsassociatedsignaling circuit, means controlled by each of said relay means forlocking said relay means in operated condition, station selector switchmeans connected with each of said communication circuits including meanscontrolling the associated relay means and operable to "a predeterminedposition for returning said relay means to a nonoperated condition, andselectively operable, means for rendered a selected oneofj said locking,circuits ineffective.

12. Communication apparatus including a plurality of circuits, relaymeans in each of said circuits operable upon momentary completion of itsassociated circuit, means controlled by each of said relay means forlooking it in operated condition, means connected with each of saidcircuits-forunlockinga locked relay means, and means operativelyconnected with all said circuits for selectively renderin any one ofsaid locking means ineffective.

13. Communication apparatus including a central station, a pair ofremote stations each having voice receiving and transmitting means,amplifying means at the central station having an input and an output,means at the central station including selectively operable switch meansfor selectively connecting said voice receiving and transmitting meansto the input and output of said amplifying means and electromagneticmeans for operating said selectively operable means, and means includinga control circuit leading to one of said remote stations and selectivelyoperable means in said circuit at said one station controlling saidelectromagnetic means for effecting two-way communication between saidremote stations.

.14. A communication system including a central station and a pluralityof remote stations, a communication circuit. between each of said remotestations and the central station, selectively operable means at thecentral station and circuits controlled thereby for establishingcommunication circuits between selected remote stations, controllablecommunication direction control means at the central station in theestablished communication circuits, and a control circuit for saidcontrollable communication direction control means including a switch atone of said remote stations for which the communication circuits areestablished.

15. Communication apparatus including a central station, a pair ofremote stations each having voice receiving and transmitting means,amplifyin means at the first station having an input and an output,means at the central station including selectively operable switch meansfor selectively connecting said voice receiving and transmitting meansto the input and output of said amplifying means and electromagneticmeans for operating said selectively operable means, and means includinga control circuit leading to one of said remote stations and selectivelyoperable means in said circuit at said one station controlling saidelectromagnetic means for effecting two-way communication between saidremote stations, second amplifying means at said central station, andmeans including selectively operable switch means and circuitscontrolled thereby for substituting said second amplifyin means in placeof said first amplifying means.

16. A communication system including a plurality of remote stations, acentral station including amplifying means, a communication circuitbetween each of said remote stations and the central station,selectively operable switch means at the central station for connectingsaid communication circuits to said amplifying means, selectivelyoperable second switch means at the central station for reversiblyconnecting said communication circuits to the input and output of saidamplifying means, and means including control means at one of saidremote stations and operating means for said second switch means at thecentral station controlled thereby for reversing the connections of saidcircuits to said amplifying means.

17. A communication system including a plurality of remote stations, acentral station includin amplifying means and two amplifiercommunication circuits, selectively operable switch means at the centralstation for reversibly connecting said amplifier communication circuitsto the input and output of said amplifying means, an interstationcommunication circuit between each of said remote stations and thecentral station, selector switches connected to each of said amplifiercommunication circuits, branch communication circuits leading from eachof said interstation communication circuits to said selector switcheswhereby said interstation circuits are selectively connectable to saidamplifier communication circuits, and means including control means atone of said remote stations and operating means for said second switchmeans at the central station controlled thereby for reversing theconnections of said circuits to said amplifying means.

18. A communication system including a plurality of remote stations, acentral station including amplifying means and two amplifiercommunication circuits, selectively operable switch means at the centralstation for reversibly con necting said amplifier communication circuitsto the input and output of said amplifying means,

an interstation communication circuit between each of said remotestations and the central station, selector switches connected to each ofsaid amplifier communication circuits, and branch communication circuitsleading from each of said interstation communication circuits to saidselector switches whereby said interstation circuits are selectivelyconnectable to said amplifier communication circuits.

19. A communication system including a central station and a pluralityof remote stations, a signaling circuit between each of said remotestations and the central station, a communication circuit between eachof said remote stations and the central station, means including asignaling switch at each remote station in its associated signalingcircuit for signaling the central station from the remote station,selectively operable means at the central station and circuit controlledthereby for establishing communication circuits between selected remotestations, controllable communication direction control means atithecentral station in the established communication circuits, and a controlcircuit for said controllable communication direction control meansincluding the signaling switch at one of said remote stations for whichthe communication circuits are established.

20. A communication system including a plurality of remote stations, acentralstation including a plurality of amplifying means, a firstcommunication circuit between each of said remote stations and thecentral station, a signaling circuit between each of said remotestations and the central station, a signaling switch in each of saidcircuits located at the remote stations, signal means at the centralstation corresponding to each remote station, relay means at the centralstation operable upon closure of a signaling switch for operating thesignal means, means operable by said relays for locking the relays inoperated position, means including station selector switches at thecentral station adapted selectively to connect said communicationcircuits to one of said amplifying means for effecting communicationbetween said central and remote stations, means operable by saidselector switches for unlocking said relays, a second communicationcircuit between each of said remote stations and said central station,means including a pair of selectively operable switches at the centralstation for connecting the second communication circuits correspondingto calling and called remote stations, respectively, to the other ofsaid amplifying means, other selectively operable switches and a relayoperating said switches at said central station for reversing theconnections of said second communication circuits to said otheramplifying means, and the signaling switch at one of said remotestations for controlling said relay operating said other selectivelyoperable switches for effecting communication between said remotestations, said central station including also switch means operablesimultaneously with the one of said pair of switches corresponding tothe calling station for opening the locking circuit of the relay meanscorresponding to the calling station upon closure of the signalingswitch at the calling remote station.

21. A communication system including a central station and a pluralityof remote stations, signaling and communication circuits at the remotestations, a signal switch at each remote station in its signalingcircuit, signal control 13 means at the central station foreach remotestation comprising a relay and a relay locking circuit closed uponoperation of the relay, relay control means at the central stationoperable to unlock said relay upon establishment of communicationbetween the central and remote stations, means at the central stationfor establishing communication between selected remote stations, meansincluding the signal switch at one of said remote stations and thecorresponding signal circuit for controlling the direction ofcommunication between said selected remote stations, and means at thecentral station comprising a switch in said locking circuit operableupon establishment of said communication circuit for preventing lockingof the relay controlled by the signal switch.

RICHARD H. CAMPBELL.

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